For aerospace applications, compositions serving as sealants and/or adhesives must possess excellent fluid resistance to a variety of fluids, including jet fuel, de-icing fluid, water, hydraulic oil, lubricating oil, and the like. Compositions must also provide excellent post-cure mechanical properties, such as high flexibility.
In addition, many compositions employed as sealants and/or adhesives must be able to be applied rapidly, with minimal man-hour and material costs, and cure without dimensional changes into a multifunctional material. Rapid application is assisted by low melting temperature and relatively low melt viscosity in a pre-cured state. In addition to fluid resistance and flexibility, post-cure performance requirements include excellent adhesion to aluminum, titanium, carbon fiber-epoxy composite, and primed and/or promoted surfaces; adhesion, mechanical strength, and dimensional stability over a wide temperature range and appropriate surface tension properties for top-coating.
Existing adhesive or sealant materials that offer similar post-cure performance include variants such as two-component (2K) flexibilized epoxies; 2K or three component (3K) polysiloxanes; 2K polysulfides or polythioethers or thermoplastic polyurethanes that are melted into place. However, epoxies, polysiloxanes, and polysulfides/thioethers all suffer from the drawbacks of 2K systems, namely extra time to mix components, mixing inconsistencies (inconsistent ratios, material degradation, and entrapped air), short pot life, and long cure time. The thermoplastic polyurethanes and similar melt-in-place thermoplastics suffer from lengthy application time and high application temperatures during the melting process, poor adhesion, and low post-application maximum operating temperatures.
It would therefore be highly desirable to achieve a sealant or adhesive material with easy application and handling while in a pre-cured state which possesses excellent fluid resistance, dimensional stability, and adhesion to a variety of surfaces in a post-cured state. It is towards meeting this need that the embodiments as disclosed herein are directed.
In general, the compositions as disclosed herein are in the form of a reactive hot melt (RHM) adhesive which includes a urethane prepolymer containing a flexible polyol, a crystallizable polyol, a high hard segment content, a filler package, a leachable plasticizer and, optionally additives. Prior to cure, the urethane prepolymer exhibits a low melting point and a melt viscosity suitable for application. During the cure process, the isocyanate groups exhibit excellent bond strength to a variety of surfaces. Once cured with exposure to moisture, the RHM adhesive material retains adhesion, shape, and density, even after exposure to polar and non-polar solvents including water, deicing fluid, hydraulic oil, JP-8, and lubricating fluids.
As an additional step, the RHM adhesive material may be mixed with a hydroxyl and/or amine functional hardener to create an elastomer. Except for adhesion to a surface, which will be modified depending on the crosslinker and the application process, all post-cure properties (fluid resistance, elasticity, etc.) are retained or enhanced by crosslinking the isocyanate groups with hydroxyl and/or amine groups. Cure proceeds immediately upon mixing with the hardener.
According to embodiments disclosed herein a reactive hot melt (RHM) composition is provided as a mixture of a urethane prepolymer which is a reaction product of at least one polyol and at least one isocyanate, a filler material, and a leachable plasticizer consisting of an aliphatic ester of a carboxylic acid.
In some embodiments, the at least one isocyanate is selected from the group consisting of hexamethylene diisocyanate (HDI), methylenediphenyl diisocyanate (MDI), tolulene diisocyanate (TDI), isophorone diisocyanate (IPDI) and hydrogenated methylenediphenyl diisocyanate (hMDI). Certain embodiments will employ a mixture of hexamethylene diisocyanate (HDI) and methylenediphenyl diisocyanate (MDI) in about a 1:1 weight ratio.
The at least one polyol is selected from the group consisting of polyester polyols, polyethylene copolymer polyols, polysiloxane polyols and acrylic polyols.
The filler material may be least one fibrous or particulate filler selected from the group consisting of silica, carbon or metallic filler materials. As exemplary, the filler material may be at least one selected from the group consisting of nickel, iron, aluminum, tungsten, silver, gold, platinum, palladium and carbon materials. The filler material may be employed in an amount between about 0.1 wt. % to about 80 wt. %, based on total composition weight.
According to some embodiments, the leachable plasticizer consists of an aliphatic ester of citric acid, malonic acid and/or phthalic acid. Examples of such plasticizers include triethyl citrate, tributyl acetylcitrate and diethyl malonate. A non-ionic miscibilizer may optionally be employed, such as ethylenediamine tetrakis(propoxylate-block-ethoxylate) tetrol (EDTP-b-ET) and poly(ethylene glycol) sorbitol hexaoleate (PEGSH). The leachable plasticizer may be present in an amount between about 1.0 wt % to about 40 wt. %.
An additive other than the filler material may be employed in other embodiments in an amount about 0.1 wt. % to about 80 wt. %. Such other optional additive may be one selected from the group consisting of density modifiers, dispersion additives, blocking agents, air reducing additives, antioxidants, flow improvers, conductivity modifiers and cure rate accelerators.
The RHM composition may be made by (a) forming a urethane prepolymer by reacting at least one polyol and at least one isocyanate, and thereafter (b) mixing the urethane prepolymer with (i) a filler material and (ii) a leachable plasticizer consisting of an aliphatic ester of a carboxylic acid.
These and other aspects of the present invention will become more clear after careful consideration is given to the following detailed description of a presently preferred exemplary embodiment thereof.